In the vibratory method of Doty et al., U.S. Pat. No. 2,688,124 Aug. 31, 1954 "Method and Apparatus for Determining Travel Time of Signals", seismic waves are generated by mechanical vibrators on the earth's surface. The waves propagate through the earth in various directions from the vibratory source. Some of the wave energy propagates on downward indefinitely and serves no useful exploration purpose, but at least part of the wave energy is scattered back toward the earth's surface by various reflecting, diffracting, and refracting subsurface formations. That part, when suitably recorded and processed serves to delineate the subsurface formations that scattered it back toward the earth's surface.
In the vibratory method, the waves sent into the earth consist of long wave trains rather than the much sharper wave pulses sent into the earth by the explosive sources used before the Doty et al. invention. An essential part of the Doty et al. invention was to process the received data to produce records that tended to show short pulses representing reflections from subsurface interfaces. The patentees produced the desired shorter pulses on their ultimate seismic records by cross-correlating a recorded representation of the vibratory waves sent into the ground with the recorded representation of the waves received subsequently.
The use of cross-correlation as taught by Doty et al. and many others since, has now become so well known in vibratory seismology that it will be presumed to be well known in the following parts of the present specification; and the description will concern itself only with differences from the prior art.
Martin et al., U.S. Pat. No. 4,058,791 issued Nov. 15, 1977 "Method and Apparatus for Processing Seismic Signals from low Energy Sources" is directed to an effort to solve the growing problem of handling all the information collected in a modern seismic survey. It is now desired to collect information from hundreds, and sometimes even thousands, of receivers, feeding into tens, and sometimes even hundreds, of recording channels. Martin et al. recognize that some essence of the seismic information is preserved if only the algebraic signs of the incoming signals, and not the full wave forms are recorded. Using information channels that need to handle only sign-bits makes it possible to use several times as many channels for the same recording and processing capacity.
Also, Martin et al. observed that in some of their vibratory seismic work, that when sign-bit representations of the source waves were cross-correlated with sign-bit representations of the received waves, the resulting cross-correlation functions appear to be similar to cross-correlation functions from full waveform inputs, provided that the resulting correlation functions are "common depth point stacked" to a high multiplicity ("the CDP fold is at least 40"). However, it is to be particularly noted that Martin et al. use a conventional "chirp" source signal to generate vibrations. Furthermore, Martin et al. indicate that where their stacked final records appeared similar to conventional stacked records using 16-bit recording, they were referring to work of their predecessors, such as Fort et al., U.S. Pat. No. 3,883,725, issued May 13, 1975, "Data Compositing and Array Control System", who added certain "shifting functions" to the received signals before the received signals were clipped. The requirement for high order stacking is objectionable because the large number of information channels required to produce a single stacked output trace tends to cancel out the very advantage for which sign-bit recording is used, its channel-capacity economy. A large number of low capacity channels can require just as much recording and processing capacity as a small number of high capacity channels. Addition of the "shifting functions" is objectionable. It does not improve records in the general case, even though it may have some value in certain limited circumstances (e.g., with low signal-to-noise-ratio signals). So the similarities noted by Martin et al. between the cross-correlation function from their sign-bit recordings and cross-correlations from full waveform recordings depended on special circumstances not desirable to produce, or to encounter, in general seismic exploration work.
There is a further disadvantage of the Martin et al. technique; they had no measure of the similarity they noticed. The results could not be stated in mathematical terms which would indicate how much information had been discarded in the clipping operation (the conversion to sign-bits) and whether or not the discarded information was essential.
Another relevant patent is that of Crook et al., U.S. Pat. No. 3,264,606, Aug. 2, 1966 "Method and Apparatus for Continuous Wave Seismic Prospecting" which teaches driving of vibratory sources (in conjunction with conventional full-wave recording equipment) with pseudo-random codes which, although differing in detail from the preferred codes prescribed here, does share the desirable generic property of "a code sequence which may be represented as a reference time series having a unique auto-correlation function comprising a single major lobe having no side lobes of greater amplitude than the side lobes of the auto-correlation function of statistically unrelated noise components of the composite signal detected at said detecting location" (Column 13, lines 32-44). Aside from the prior art patents, the most relevant technical reference may be the paper of A. B. Cunningham, Geophysics, December 1979, Vol. 44, No. 12, pages 1901 et seq. The Cunningham paper entitled "Some Alternate Vibrator Signals" worked out in mathematical detail the expected types of cross-correlation functions from various types of vibrator sweeps, including certain types of pseudo-random sweeps.